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第八章 IO库

8.1&&8.2

 1 #include <iostream>
 2 #include <vector> 
 3 #include <string>
 4  
 5 using namespace std;
 6 
 7 istream &func(istream &is)
 8 {
 9     int val;
10     while (is >> val && !is.eof()) {
11         cout << val << endl;
12     }
13     is.clear();        //复位所有错误标志位 
14 }
15 
16 int main()
17 {
18     func(cin);
19     return 0;
20 }

View Code

8.3

读取类型不匹配；遇到文件结束符EOF；IO流错误

8.4

 1 #include <iostream>
 2 #include <fstream> 
 3 #include <vector> 
 4 #include <string>
 5  
 6 using namespace std;
 7 
 8 int main()
 9 {
10     vector<string> vec;
11     string ifile = "data.txt", ss;    //需在本文件夹下有data.txt这个文件 
12     ifstream in(ifile);
13     while(getline(in, ss)) {
14         vec.push_back(ss);
15     }
16     in.close();
17     for (auto i : vec)
18         cout << i << endl;
19     return 0;
20 }

View Code

8.5

 1 #include <iostream>
 2 #include <fstream> 
 3 #include <vector> 
 4 #include <string>
 5  
 6 using namespace std;
 7 
 8 int main()
 9 {
10     vector<string> vec;
11     string ifile = "data.txt", ss;    //需在本文件夹下有data.txt这个文件 
12     ifstream in(ifile);
13     while(in >> ss) {            //改动处 
14         vec.push_back(ss);
15     }
16     in.close();
17     for (auto i : vec)
18         cout << i << endl;
19     return 0;
20 }

View Code

8.6

 1 #include <iostream>
 2 #include <fstream> 
 3 #include <vector> 
 4 #include <string>
 5  
 6 using namespace std;
 7 
 8 struct Sales_data {
 9     string bookNo;                //书的ISBN 
10     unsigned units_sold = 0;    //售出的本数 
11     double revenue = 0.0;        //销售额 
12     Sales_data& combine(const Sales_data &);
13     string isbn()    {    return bookNo;    }
14 };
15 
16 Sales_data &Sales_data::combine(const Sales_data &rhs)
17 {
18     units_sold += rhs.units_sold;
19     revenue += rhs.revenue;
20     return *this;
21 }
22 
23 istream &read(istream &is, Sales_data &item)
24 {
25     is >> item.bookNo >> item.units_sold >> item.revenue;
26     return is;
27 }
28 
29 ostream &print(ostream &os, const Sales_data &item)
30 {
31     os << item.bookNo << " " << item.units_sold << " " << item.revenue;
32     return os;
33 }
34 
35 Sales_data add(const Sales_data &lhs, const Sales_data &rhs)
36 {
37     Sales_data sum = lhs;
38     sum.combine(rhs);
39     return sum;
40 }
41 
42 int main(int argc, char *argv[])
43 {
44     if (argc != 2) {  
45         cerr<< "Please give the file name."<<endl;  
46         return -1;  
47     }
48     ifstream in(argv[1]);
49     if (!in) {  
50         cerr<<"Can't open the file."<<endl;  
51         return -1;  
52     }
53     Sales_data total;
54     if (read(in, total)) {
55         Sales_data trans;
56         while (read(in, trans)) {
57             if (total.isbn() == trans.isbn())    total = add(total, trans);
58             else {
59                 print(cout, total);
60                 cout << endl;
61                 total = trans;        //结构体赋值很方便 
62             }
63         }
64         cout << total.bookNo << " " << total.units_sold << " " << total.revenue << endl;
65     }
66     else {
67         cerr << "No data?!" << endl;
68         return -1;
69     }
70     return 0;
71 }

View Code

文件名（book.txt）通过参数传给main函数：

8.7

 1 #include <iostream>
 2 #include <fstream> 
 3 #include <vector> 
 4 #include <string>
 5  
 6 using namespace std;
 7 
 8 struct Sales_data {
 9     string bookNo;                //书的ISBN 
10     unsigned units_sold = 0;    //售出的本数 
11     double revenue = 0.0;        //销售额 
12     Sales_data& combine(const Sales_data &);
13     string isbn()    {    return bookNo;    }
14 };
15 
16 Sales_data &Sales_data::combine(const Sales_data &rhs)
17 {
18     units_sold += rhs.units_sold;
19     revenue += rhs.revenue;
20     return *this;
21 }
22 
23 istream &read(istream &is, Sales_data &item)
24 {
25     is >> item.bookNo >> item.units_sold >> item.revenue;
26     return is;
27 }
28 
29 ostream &print(ostream &os, const Sales_data &item)
30 {
31     os << item.bookNo << " " << item.units_sold << " " << item.revenue;
32     return os;
33 }
34 
35 Sales_data add(const Sales_data &lhs, const Sales_data &rhs)
36 {
37     Sales_data sum = lhs;
38     sum.combine(rhs);
39     return sum;
40 }
41 
42 int main(int argc, char *argv[])
43 {
44     if (argc != 3) {  
45         cerr<< "Please give the input file name and output file name."<<endl;  
46         return -1;  
47     }
48     ifstream in(argv[1]);
49     ofstream out(argv[2]);
50     if (!in) {  
51         cerr<<"Can't open the file."<<endl;  
52         return -1;  
53     }
54     if (!out) {  
55         cerr<<"can't open output file."<<endl;  
56         return -1;  
57     } 
58     Sales_data total;
59     if (read(in, total)) {
60         Sales_data trans;
61         while (read(in, trans)) {
62             if (total.isbn() == trans.isbn())    total = add(total, trans);
63             else {
64                 print(out, total) << endl;        //输出到指定文件中 
65                 total = trans;
66             }
67         }
68         print(out, total) << endl;
69     }
70     else {
71         cerr << "No data?!" << endl;
72         return -1;
73     }
74     return 0;
75 }

View Code

将上一题输出的内容输出到文件book1.txt中：

8.8

 1 #include <iostream>
 2 #include <fstream> 
 3 #include <vector> 
 4 #include <string>
 5  
 6 using namespace std;
 7 
 8 struct Sales_data {
 9     string bookNo;                //书的ISBN 
10     unsigned units_sold = 0;    //售出的本数 
11     double revenue = 0.0;        //销售额 
12     Sales_data& combine(const Sales_data &);
13     string isbn()    {    return bookNo;    }
14 };
15 
16 Sales_data &Sales_data::combine(const Sales_data &rhs)
17 {
18     units_sold += rhs.units_sold;
19     revenue += rhs.revenue;
20     return *this;
21 }
22 
23 istream &read(istream &is, Sales_data &item)
24 {
25     is >> item.bookNo >> item.units_sold >> item.revenue;
26     return is;
27 }
28 
29 ostream &print(ostream &os, const Sales_data &item)
30 {
31     os << item.bookNo << " " << item.units_sold << " " << item.revenue;
32     return os;
33 }
34 
35 Sales_data add(const Sales_data &lhs, const Sales_data &rhs)
36 {
37     Sales_data sum = lhs;
38     sum.combine(rhs);
39     return sum;
40 }
41 
42 int main(int argc, char *argv[])
43 {
44     if (argc != 3) {  
45         cerr<< "Please give the input file name and output file name."<<endl;  
46         return -1;  
47     }
48     ifstream in(argv[1]);
49     ofstream out(argv[2], ofstream::app);        //唯一改变之处 
50     if (!in) {  
51         cerr<<"Can't open the file."<<endl;  
52         return -1;  
53     }
54     if (!out) {  
55         cerr<<"can't open output file."<<endl;  
56         return -1;  
57     } 
58     Sales_data total;
59     if (read(in, total)) {
60         Sales_data trans;
61         while (read(in, trans)) {
62             if (total.isbn() == trans.isbn())    total = add(total, trans);
63             else {
64                 print(out, total) << endl;
65                 total = trans;
66             }
67         }
68         print(out, total) << endl;
69     }
70     else {
71         cerr << "No data?!" << endl;
72         return -1;
73     }
74     return 0;
75 }

View Code

8.9

 1 #include <iostream>
 2 #include <vector> 
 3 #include <string>
 4 #include <sstream> 
 5  
 6 using namespace std;
 7 
 8 int main()
 9 {
10     string line, word;    
11     while(getline(cin, line)) { 
12         istringstream in(line);
13         while (in >> word) 
14             cout << word << endl;
15     }
16     return 0;
17 }

View Code

8.10

 1 #include <iostream>
 2 #include <vector> 
 3 #include <string>
 4 #include <fstream> 
 5 #include <sstream> 
 6  
 7 using namespace std;
 8 
 9 int main()
10 {
11     vector<string> vec;
12     string line, word;
13     ifstream input("data.txt");
14     while(getline(input, line)) { 
15         vec.push_back(line);
16     }
17     for (auto i : vec) {
18         istringstream in(i);
19         while (in >> word) {
20             cout << word << endl;
21         }    
22     }
23     return 0;
24 }

View Code

8.11

 1 #include <iostream>
 2 #include <vector> 
 3 #include <string>
 4 #include <fstream> 
 5 #include <sstream> 
 6  
 7 using namespace std;
 8 
 9 struct PersonInfo {  
10     string name;  
11     vector<string> phones;  
12 };  
13   
14 int main()  
15 {  
16     string line, word;  
17     vector<PersonInfo> people;  
18     istringstream record;      //定义在循环之外 
19     while (getline(cin,line)) {   
20         PersonInfo info;  
21         record.clear();      //每次要解除上次绑定的string对象 
22         record.str(line);  
23         record >> info.name;  
24         while (record >> word)  
25             info.phones.push_back(word);  
26         people.push_back(info);  
27     }  
28     return 0;  
29 }

View Code

8.12

因为此时我们需要其中的成员皆可被访问，所以 PersonInfo 是一个聚合类，不能在内部进行初始化

8.13

 1 #include <iostream>
 2 #include <vector> 
 3 #include <string>
 4 #include <fstream> 
 5 #include <sstream> 
 6  
 7 using namespace std;
 8 
 9 struct PersonInfo {  
10     string name;  
11     vector<string> phones;  
12 };  
13   
14 int main()  
15 {  
16     ifstream in("data.txt");         //从文件读取 
17     string line, word;  
18     vector<PersonInfo> people;  
19     istringstream record;      
20     while (getline(in,line)) {      //in为文件输入流 
21         PersonInfo info;  
22         record.clear();  
23         record.str(line);  
24         record >> info.name;  
25         while (record >> word)  
26             info.phones.push_back(word);  
27         people.push_back(info);  
28     }  
29     return 0;  
30 }

View Code

8.14

这两条语句分别适用范围for语句枚举people中所有项和每项的phones中的所有项。

使用const表明在循环中不会改变这些项的值；

使用auto是请求编译器依据vector元素类型来推断出entry和nums的类型，既简化代码又避免出错；

使用引用的原因是，people和phones的元素分别是结构对象和字符串对象，使用引用即可避免对象拷贝。

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原文地址：https://www.cnblogs.com/xzxl/p/7681844.html